Vacuum leaks can be as simple as a bad vacuum wiper motor, or a bad seal between the intake manifold and the head, or sloppy throttle shaft holes.
Or a little more invasive like bad valves/ seats or worse yet piston rings.
If you dont already have buy a vacuum gauge and learn how to use it as a diagnostic tool. Reading and needle fluctuations can mean a whole host of things to point the finger too. Probably the most valuable diagnostic tool in the tool chest.
